I. Commercial Melting Furnaces
A large percentage of iron produced in the United States is melted in cupola-type furnaces. A large percentage of steel is processed from melting steel scrap by means of electric arc furnaces in mini-steel mills.
In a typical cupola, a bed of coke is laid on the bottom of a shaft type furnace and this coke, which is hot, is ignited and combusted by blowing air through it. Alternate layers of scrap, iron and coke are fed to the cupola through the top and the heat produced by combustion of the coke melts the scrap. Molten iron collected at the bottom is generally tapped at the side of the furnace. This method of iron production uses coke i) as a fuel, ii) as a source of carbon which is contained within the iron, and iii) as a mechanism for generating a reducing atmosphere.
In an electric melting furnace, a large electric current is passed through graphite electrodes to generate an arc which melts the scrap charged as a batch into the furnace. The following discussion applies to all electric furnaces whether of the direct-arc type where heat is provided by an arc passing electric current through the charge or where the heat is provided by an arc between electrodes as in an indirect-arc furnace or where the heat is provided by an arc confined for concentrated heating by an electromagnetic field such as in a plasma-arc furnace.
Apart from historical reasons, it is believed the cupola has enjoyed long-term commercial success because it is simple and continuous and, as noted, the coke generates a reducing or protective atmosphere which adds carbon to the metal lowering the melt temperature of the iron. For steel, it is believed that the electric furnace has enjoyed commercial success because extremely high temperatures are produced at the electrode which more than compensate for any oxidation which might occur while the high temperatures produce a very fast throughput heating time for the charge. Nevertheless, both cupola and electric furnaces have drawbacks when they are analyzed against the basic objectives of a metal melting system or furnace. These objectives may be set forth as follow:
a) Basic Process Operation
The presence of oxygen during melting of metal will produce metal oxides. This is, of course, deleterious to the metal making operation. The oxides must either be separated and disposed of or additional heat applied to break the oxide bond.
In short, metal oxidation during melting is expensive and creates a waste which must be handled and disposed of. As already noted, in the electric arc furnace the carbon electrodes are at such a high temperature that they easily react with any oxidant inside the melter as long as the melter vessel is kept tight and oxygen or air is prevented from leaking into the melter. In the cupola, coke is mixed between the scrap and this carbon source scavenges all oxidants above a certain temperature. Technically, it is the ability of the cupola and the electric arc furnace to dissipate the metal oxidation problem which has resulted in the commercial success of the two processes.
In addition to preventing metal oxidation, control of the final melt temperature in a melting furnace is very important for many liquid metal operations. In any casting operation, a certain amount of superheat within the melt must be available to prevent premature freezing in ladling and casting operations. This metal superheat is easily accomplished in an electric arc furnace. It cannot be as readily accomplished in the cupola. Cupolas, therefore, have holding furnaces often attached to them which are used to adjust metal composition. The holding furnaces have to be continually heated before tapping.
A further basic requirement of a melting furnace is that the melter must be capable of operating at high production rates. This can be accomplished in the electric arc furnace by simply building larger arc furnaces. However, large melting rates translate into large utility requirements. Arc furnaces operate on low voltages. Line voltage must be transformed to lower values and the majority of arc furnaces still operate on direct current. The auxiliary electric equipment is large and expensive and contributes to a major reduction in energy efficiency. The equipment also consumes a significant amount of cooling water. In cupola operations, the significant expenses related to off gas cleaning and the essentially continuous operation of the cupola basically prevent upgrading the throughput capacity.
b) Scrap Grade and Melter Operation
Scrap is traded in several grades which have different utility in melting operations. Major scrap properties relate to typical scrap dimensions, cleanliness, composition and physical attributes like springiness and stampings. A good melter must be capable of handling a wide variety of scrap. Buying less expensive scrap grades can reduce product cost provided melting rate, product composition and melter emissions are not adversely affected. Neither the electric arc melter nor the cupola can melt one of the best suited scrap types which is turnings and borings. In addition to the scrap price, the physical attributes of various scrap, such as permeability of the scrap for the cupola and cleanliness of the scrap for the electric melter, have a major impact on melter performance and operation. The cleanliness of the scrap can be of a major concern to the safe operation of the electric arc furnace. It is known apart from any health problems or polluting problems which may relate to oily type scrap, that water trapped within scrap bundles of waste can cause the bundles to literally explode when the scrap bundles are heated at high rates by the electrode and the scrap bundles can be and have been propelled as projectiles through the electric arc furnace.
c) Metal Composition Adjustment
The melting furnace must provide for adjustment to the chemical composition of various heats produced in the mill or foundry. The electric arc furnace is ideally suited for this since it is a batch furnace. The cupola, on the other hand, must use or add an electrically heated holding furnace to permit composition adjustment and superheat control.
d) Control of Emissions
The emissions from melters can be classified into three groups which basically reflect three different groupings of contaminants in the scrap. The "as bought" scrap is dirty and contains significant amounts of oils, paints, other combustibles and water. During melting of the scrap other emissions can be produced by chemical reactions occurring in the furnace atmosphere. In the electric arc furnace, the local temperatures are so high as to produce metal vapors, especially for some lower melting heavy metals. These vapors are emitted together with iron oxides and are in the form of aerosols which makes their collection difficult. The electric arc furnace also emits large amounts of air pollutants in the form of unburned hydrocarbons from the heating of dirty scrap and carbon monoxide from the oxidation of the carbon electrodes which occur during melting. Thus, an afterburner and a bag house are needed to clean the exhaust gases. Even the addition of a bag house and an afterburner does not entirely solve the emission problems associated with the electric arc furnace because the electric arc furnace is a batch furnace and it is difficult to exhaust all the unburned hydrocarbons and carbon monoxides from the furnace to the incinerator.
The operation of the cupola is somewhat different due to the different nature of the melting process used in the cupola. However, the same type of exhaust gas cleaning equipment, i.e. an afterburner and a bag house, must be added to the cupola.
e) Maintenance
Both the electric arc furnace and the cupola have continuing maintenance problems. The frequency of the maintenance for both melters has been reduced due to the introduction of high temperature and wear resistant materials. However, inherent in the operation of the electric furnace is the erratic behavior of the electric arc which creates very high local heat fluxes and contributes to the early failures of refractories. To prevent such failures, modern electric arc furnaces use water cooled metal surfaces to avoid related material deterioration. This obviously results in a trade-off which is increased energy consumption to compensate for the metal cooling which not only leads to higher operating costs, but also to a larger and higher rated electric power supply system.
f) Costs--Equipment and Operating
The cupola was at one time an inexpensive piece of equipment. However, the addition of the particulate cleaning equipment and the afterburner has increased the cost of the cupola dramatically. The electric arc furnace, as already indicated, carries the additional burden of a very large, complex and expensive electrical power supply system which is in fact higher than the actual cost of the melter itself. Electrical energy costs are reflected not only in the total kilowatt usage but also in the fact that peak line power with increased energy costs must be used at various times. Such factors result in significant energy costs which will be discussed in further detail in the Detailed Description of the specifications which follows.